Well, I guess it’s time to quote this:
I was waiting all day to do this 
This is the reason I’m annoyed that the “self-identification thing” was never tried out since 2017. Essentially Glicko mostly solves that problem, most likely (of having moderators forced to manually correct ranks), but ever since Glicko has been here, self-identification has never been allowed.
So we can’t know how bad it would get because it’s never been tried, but there are good reasons to believe it wouldn’t be anywhere nearly as bad as pre-2017 OGS.
Though to be fair these 5 years have been good to partially clean OGS’s reputation of being sandbagger hell.
By the way, to be clear, you don’t even need to do anything to the rating system per se. The initial hidden rank can be 6 kyu as much as you want: as soon as a provisional player loses a game against a TPK, their rank estimate will plummet down there, so you just need to match beginners against weak players for the first few games.
Same for dan players, as soon as they win a game against a high SDK or a dan, their rank will jump up. If the self-identified dan rank was wrong, they will lose against the strong players, and their rank will stay near 6 kyu and uncertain, and Glicko will work its “magic”.
This does allow for intentional sandbaggers to intentionally lose the first few games, though, but then again they can do that with the current system too.
If you’re worried, you can just implement the new system gradually, to see how many self-identified newcomers we can handle.First, you keep handling most newcomers with the current system and you only offer the quick questionnaire to a few randomly selected newcomers, say 10 every day.
Then you gradually increase the number of self-identified newcomers until either problems start to arise or you’re handling all newcomers that way, which means you’ve proven that the self-identifying system doesn’t cause problems after all.If there are problems, we may start to consider something like the “beginner supervised games” system that I described here, or an improved version of it, or a completely different proposal. But that comes after verifying that there are problems.
There's one last kink to maybe worry about, though not critical:
To avoid annoying players of a specific rank in the established OGS population with provisional players, like OP here was complaining about, just implement a more “fuzzy” matchmaking system: instead of allowing players to select a specific rank to start from, only allow them ranges, and either assign them a randomly selected rating from the range for matchmaking purposes, or just matchmake them with the whole range and match them with the first player that pops up.
That way, there’s no specific rank being forced to handle all the newcomers, like the 12 kyu players are in the current system.
Just to be clear, I believe that implementing the self-identifying proposal would require fiddling with the matchmaking system anyway, at least if implemented the way I proposed above, so I think this would not be that much more work.
Well, at some point I’d like to hear what the devs think about this assortment of ideas, but I never know when it’s a good time to ping them and ask them directly. For now, at the very least I can wait to see if somebody can (or is willing to) offer good counter-arguments here, though most people never read my long replies